The alloca-opt module provides for a function alloca which allocates
memory on the stack, where the system allows it. A memory block allocated with
alloca exists only until the function that calls alloca returns
or exits abruptly.
There are a few systems where this is not possible: HP-UX systems, and some
other platforms when the C++ compiler is used. On these platforms the
alloca-opt module provides no replacement, just a preprocessor macro
HAVE_ALLOCA.
The user can #include <alloca.h> on all platforms, and use
alloca on those platforms where the preprocessor macro HAVE_ALLOCA
evaluates to true. If HAVE_ALLOCA is false, the code should use a heap-based
memory allocation based on malloc or (in C++) new. Note that
the #include <alloca.h> must be the first one after the
autoconf-generated config.h, for AIX 3 compatibility. Thanks to IBM for
this nice restriction!
Note that GCC 3.1 and 3.2 can inline functions that call alloca.
When this happens, the memory blocks allocated with alloca will not be
freed until the end of the calling function. If this calling function
runs a loop calling the function that uses alloca, the program easily
gets a stack overflow and crashes. To protect against this compiler behaviour,
you can mark the function that uses alloca with the following attribute:
#ifdef __GNUC__ __attribute__ ((__noinline__)) #endif